She was wearing a red shirt in Don's near death experience/drug-induced hallucination. We know what that means.
But seriously, the more I think about it, the more I think the theory of Megan dying may be correct. In fact, I'm leaning toward this
theory that's she already dead and Don just hasn't discovered it yet. It is consistent with the fact that, at the end of the last episode Don returned from CA and went straight to the office.